1Using RCU's CPU Stall Detector 2 3The rcu_cpu_stall_suppress module parameter enables RCU's CPU stall 4detector, which detects conditions that unduly delay RCU grace periods. 5This module parameter enables CPU stall detection by default, but 6may be overridden via boot-time parameter or at runtime via sysfs. 7The stall detector's idea of what constitutes "unduly delayed" is 8controlled by a set of kernel configuration variables and cpp macros: 9 10CONFIG_RCU_CPU_STALL_TIMEOUT 11 12 This kernel configuration parameter defines the period of time 13 that RCU will wait from the beginning of a grace period until it 14 issues an RCU CPU stall warning. This time period is normally 15 21 seconds. 16 17 This configuration parameter may be changed at runtime via the 18 /sys/module/rcupdate/parameters/rcu_cpu_stall_timeout, however 19 this parameter is checked only at the beginning of a cycle. 20 So if you are 10 seconds into a 40-second stall, setting this 21 sysfs parameter to (say) five will shorten the timeout for the 22 -next- stall, or the following warning for the current stall 23 (assuming the stall lasts long enough). It will not affect the 24 timing of the next warning for the current stall. 25 26 Stall-warning messages may be enabled and disabled completely via 27 /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress. 28 29CONFIG_RCU_CPU_STALL_INFO 30 31 This kernel configuration parameter causes the stall warning to 32 print out additional per-CPU diagnostic information, including 33 information on scheduling-clock ticks and RCU's idle-CPU tracking. 34 35RCU_STALL_DELAY_DELTA 36 37 Although the lockdep facility is extremely useful, it does add 38 some overhead. Therefore, under CONFIG_PROVE_RCU, the 39 RCU_STALL_DELAY_DELTA macro allows five extra seconds before 40 giving an RCU CPU stall warning message. (This is a cpp 41 macro, not a kernel configuration parameter.) 42 43RCU_STALL_RAT_DELAY 44 45 The CPU stall detector tries to make the offending CPU print its 46 own warnings, as this often gives better-quality stack traces. 47 However, if the offending CPU does not detect its own stall in 48 the number of jiffies specified by RCU_STALL_RAT_DELAY, then 49 some other CPU will complain. This delay is normally set to 50 two jiffies. (This is a cpp macro, not a kernel configuration 51 parameter.) 52 53rcupdate.rcu_task_stall_timeout 54 55 This boot/sysfs parameter controls the RCU-tasks stall warning 56 interval. A value of zero or less suppresses RCU-tasks stall 57 warnings. A positive value sets the stall-warning interval 58 in jiffies. An RCU-tasks stall warning starts wtih the line: 59 60 INFO: rcu_tasks detected stalls on tasks: 61 62 And continues with the output of sched_show_task() for each 63 task stalling the current RCU-tasks grace period. 64 65For non-RCU-tasks flavors of RCU, when a CPU detects that it is stalling, 66it will print a message similar to the following: 67 68INFO: rcu_sched_state detected stall on CPU 5 (t=2500 jiffies) 69 70This message indicates that CPU 5 detected that it was causing a stall, 71and that the stall was affecting RCU-sched. This message will normally be 72followed by a stack dump of the offending CPU. On TREE_RCU kernel builds, 73RCU and RCU-sched are implemented by the same underlying mechanism, 74while on PREEMPT_RCU kernel builds, RCU is instead implemented 75by rcu_preempt_state. 76 77On the other hand, if the offending CPU fails to print out a stall-warning 78message quickly enough, some other CPU will print a message similar to 79the following: 80 81INFO: rcu_bh_state detected stalls on CPUs/tasks: { 3 5 } (detected by 2, 2502 jiffies) 82 83This message indicates that CPU 2 detected that CPUs 3 and 5 were both 84causing stalls, and that the stall was affecting RCU-bh. This message 85will normally be followed by stack dumps for each CPU. Please note that 86PREEMPT_RCU builds can be stalled by tasks as well as by CPUs, 87and that the tasks will be indicated by PID, for example, "P3421". 88It is even possible for a rcu_preempt_state stall to be caused by both 89CPUs -and- tasks, in which case the offending CPUs and tasks will all 90be called out in the list. 91 92Finally, if the grace period ends just as the stall warning starts 93printing, there will be a spurious stall-warning message: 94 95INFO: rcu_bh_state detected stalls on CPUs/tasks: { } (detected by 4, 2502 jiffies) 96 97This is rare, but does happen from time to time in real life. It is also 98possible for a zero-jiffy stall to be flagged in this case, depending 99on how the stall warning and the grace-period initialization happen to 100interact. Please note that it is not possible to entirely eliminate this 101sort of false positive without resorting to things like stop_machine(), 102which is overkill for this sort of problem. 103 104If the CONFIG_RCU_CPU_STALL_INFO kernel configuration parameter is set, 105more information is printed with the stall-warning message, for example: 106 107 INFO: rcu_preempt detected stall on CPU 108 0: (63959 ticks this GP) idle=241/3fffffffffffffff/0 softirq=82/543 109 (t=65000 jiffies) 110 111In kernels with CONFIG_RCU_FAST_NO_HZ, even more information is 112printed: 113 114 INFO: rcu_preempt detected stall on CPU 115 0: (64628 ticks this GP) idle=dd5/3fffffffffffffff/0 softirq=82/543 last_accelerate: a345/d342 nonlazy_posted: 25 .D 116 (t=65000 jiffies) 117 118The "(64628 ticks this GP)" indicates that this CPU has taken more 119than 64,000 scheduling-clock interrupts during the current stalled 120grace period. If the CPU was not yet aware of the current grace 121period (for example, if it was offline), then this part of the message 122indicates how many grace periods behind the CPU is. 123 124The "idle=" portion of the message prints the dyntick-idle state. 125The hex number before the first "/" is the low-order 12 bits of the 126dynticks counter, which will have an even-numbered value if the CPU is 127in dyntick-idle mode and an odd-numbered value otherwise. The hex 128number between the two "/"s is the value of the nesting, which will 129be a small positive number if in the idle loop and a very large positive 130number (as shown above) otherwise. 131 132The "softirq=" portion of the message tracks the number of RCU softirq 133handlers that the stalled CPU has executed. The number before the "/" 134is the number that had executed since boot at the time that this CPU 135last noted the beginning of a grace period, which might be the current 136(stalled) grace period, or it might be some earlier grace period (for 137example, if the CPU might have been in dyntick-idle mode for an extended 138time period. The number after the "/" is the number that have executed 139since boot until the current time. If this latter number stays constant 140across repeated stall-warning messages, it is possible that RCU's softirq 141handlers are no longer able to execute on this CPU. This can happen if 142the stalled CPU is spinning with interrupts are disabled, or, in -rt 143kernels, if a high-priority process is starving RCU's softirq handler. 144 145For CONFIG_RCU_FAST_NO_HZ kernels, the "last_accelerate:" prints the 146low-order 16 bits (in hex) of the jiffies counter when this CPU last 147invoked rcu_try_advance_all_cbs() from rcu_needs_cpu() or last invoked 148rcu_accelerate_cbs() from rcu_prepare_for_idle(). The "nonlazy_posted:" 149prints the number of non-lazy callbacks posted since the last call to 150rcu_needs_cpu(). Finally, an "L" indicates that there are currently 151no non-lazy callbacks ("." is printed otherwise, as shown above) and 152"D" indicates that dyntick-idle processing is enabled ("." is printed 153otherwise, for example, if disabled via the "nohz=" kernel boot parameter). 154 155If the relevant grace-period kthread has been unable to run prior to 156the stall warning, the following additional line is printed: 157 158 rcu_preempt kthread starved for 2023 jiffies! 159 160Starving the grace-period kthreads of CPU time can of course result in 161RCU CPU stall warnings even when all CPUs and tasks have passed through 162the required quiescent states. 163 164 165Multiple Warnings From One Stall 166 167If a stall lasts long enough, multiple stall-warning messages will be 168printed for it. The second and subsequent messages are printed at 169longer intervals, so that the time between (say) the first and second 170message will be about three times the interval between the beginning 171of the stall and the first message. 172 173 174What Causes RCU CPU Stall Warnings? 175 176So your kernel printed an RCU CPU stall warning. The next question is 177"What caused it?" The following problems can result in RCU CPU stall 178warnings: 179 180o A CPU looping in an RCU read-side critical section. 181 182o A CPU looping with interrupts disabled. This condition can 183 result in RCU-sched and RCU-bh stalls. 184 185o A CPU looping with preemption disabled. This condition can 186 result in RCU-sched stalls and, if ksoftirqd is in use, RCU-bh 187 stalls. 188 189o A CPU looping with bottom halves disabled. This condition can 190 result in RCU-sched and RCU-bh stalls. 191 192o For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the 193 kernel without invoking schedule(). Note that cond_resched() 194 does not necessarily prevent RCU CPU stall warnings. Therefore, 195 if the looping in the kernel is really expected and desirable 196 behavior, you might need to replace some of the cond_resched() 197 calls with calls to cond_resched_rcu_qs(). 198 199o Anything that prevents RCU's grace-period kthreads from running. 200 This can result in the "All QSes seen" console-log message. 201 This message will include information on when the kthread last 202 ran and how often it should be expected to run. 203 204o A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might 205 happen to preempt a low-priority task in the middle of an RCU 206 read-side critical section. This is especially damaging if 207 that low-priority task is not permitted to run on any other CPU, 208 in which case the next RCU grace period can never complete, which 209 will eventually cause the system to run out of memory and hang. 210 While the system is in the process of running itself out of 211 memory, you might see stall-warning messages. 212 213o A CPU-bound real-time task in a CONFIG_PREEMPT_RT kernel that 214 is running at a higher priority than the RCU softirq threads. 215 This will prevent RCU callbacks from ever being invoked, 216 and in a CONFIG_PREEMPT_RCU kernel will further prevent 217 RCU grace periods from ever completing. Either way, the 218 system will eventually run out of memory and hang. In the 219 CONFIG_PREEMPT_RCU case, you might see stall-warning 220 messages. 221 222o A hardware or software issue shuts off the scheduler-clock 223 interrupt on a CPU that is not in dyntick-idle mode. This 224 problem really has happened, and seems to be most likely to 225 result in RCU CPU stall warnings for CONFIG_NO_HZ_COMMON=n kernels. 226 227o A bug in the RCU implementation. 228 229o A hardware failure. This is quite unlikely, but has occurred 230 at least once in real life. A CPU failed in a running system, 231 becoming unresponsive, but not causing an immediate crash. 232 This resulted in a series of RCU CPU stall warnings, eventually 233 leading the realization that the CPU had failed. 234 235The RCU, RCU-sched, RCU-bh, and RCU-tasks implementations have CPU stall 236warning. Note that SRCU does -not- have CPU stall warnings. Please note 237that RCU only detects CPU stalls when there is a grace period in progress. 238No grace period, no CPU stall warnings. 239 240To diagnose the cause of the stall, inspect the stack traces. 241The offending function will usually be near the top of the stack. 242If you have a series of stall warnings from a single extended stall, 243comparing the stack traces can often help determine where the stall 244is occurring, which will usually be in the function nearest the top of 245that portion of the stack which remains the same from trace to trace. 246If you can reliably trigger the stall, ftrace can be quite helpful. 247 248RCU bugs can often be debugged with the help of CONFIG_RCU_TRACE 249and with RCU's event tracing. For information on RCU's event tracing, 250see include/trace/events/rcu.h. 251